This invention relates to a procedure for the purification of sewage effluent through chemical oxidation with nascent oxygen coming from the catalytic decomposition of hydrogen peroxide in continuous reaction with an iodide/iodine pair, or vice-versa.
The Spanish health authorities, as well as those of other countries, have taken the responsibility of purifying sewage before it flows into the sea and natural and artifical water courses, the sewage being derived, from human activites domestic or industrial, the sewage coming from urban areas as well as from unitary or separate sewage systems.
All the current sewage purification processes follow a scientifically established sequential model so that the purification is effective and efficient. This model is as follows:
1. Pretreatment (preliminary treatment) PA1 2. Primary treatment PA1 3. Secondary treatment PA1 4. Tertiary treatment PA1 5. Disinfection PA1 (a) Ionic exchange, in which a specific anion or cation is fixed and therefore eliminated from the sewage by passing it through a specific resin. PA1 (b) Adsorption, a physical-chemical surface phenomenon by which certain products with large specific surfaces retain other products in the sewage when they come into contact with the adsorbent product. PA1 (c) Reverse osmosis which is accomplished by applying pressure to a solution in contact with a semipermeable membrane by which the solvent, but not the dissolved, material passes.
Each one of these stages can be, in turn, of various types.
A brief explanation of the above mentioned stages is as follows:
1. Pretreatment (also called preliminary treatment)
Its goal is to eliminate coarse substances, whether floating or not, solids in suspension, sand, oils and non-emulsified greases, which can be accomplished by:
(a) Screening, consisting of passing the sewage through a grille or sieve, having a passage size smaller than the particles to be eliminated.
(b) A sand remover in order to eliminate (by small water flow velocity) the solid substances which settle in the bottom of the pretreatment vessel.
2. Primary treatment
Its goal is the elimination of solids in suspension.
This can be by:
(a) Sedimentation, which reduces the circulation velocity of the sewage until it reaches a value sufficiently low to allow deposition of sedimentary substances in the bottom of the sedimentation basin.
(b) Flocculation/coagulation
As it is known, flocculation is the formation of flocculus by the union of particles in suspension extant in a liquid, while coagulation is flocculation formed by addition of chemical products which are usually called coagulants. Examples are: Alum: (SO.sub.4).sub.3 Al.sub.2.18H.sub.2 O; Sodium aluminate: Al.sub.2 O.sub.4 Na.sub.2 ; Ferrous sulfate: SO.sub.4 Fe.7H.sub.2 O, and others known to those skilled in the art.
(c) Filtration, consisting of passing a fluid, which contains substances in suspension, through a filter medium which allows the fluid to pass through, but not the solid particulates, which are retained in the filter medium. It is an operation which is complementary to coagulation and sedimentation.
The materials most frequently used as filter media are sand, anthracite and diatomaceous earth.
3. Secondary treatment
Its goal is the elimination of soluble inorganic salts and dissolved or partially dissolved organic substances. For this it is necessary that there be a growth of microorganisms (bacteria, fungi, algae, protozoa, rotifera, etc.) with the external contribution of oxygen (aerobic process) or without it (anaerobic process).
With the addition of the mass of microorganisms (also called biomass or biological mass) the dissolved salts, nutrients and organic substance of the sewage is incorporated within the cellular content of the biomass thus reducing its concentration in the sewage.
When oxygen is required (aerobic process), the secondary treatment is known as biological oxidation.
This secondary treatment can be accomplished by:
(a) Stabilization ponds (also called stabilization lagoons).
Such ponds are constructed on the ground at a depth of 1 to 3 meters, and they constitute the simplest process of biological oxidation although they occupy very large areas of land.
They are divided into three categories:
Aerobic ponds: they rely on algae for the supply of oxygen and therefore on sun light. Their depth should be no more than 1.5 meters. For effective purification, sewage retention should be from 2 to 6 days.
Facultative ponds: they function aerobically on the surface and anaerobically at the bottom. Its effective retention time is from 7 to 30 days.
Anaerobic ponds: they are those in which the level of organic substances contained in the sewage is so high that anaerobic conditions are maintained throughout the volume of the liquid. Its effective retention time is from 30 to 50 days.
(b) Aerated lagoons (also called oxidation basins)
They are similar to the stabilization ponds except in the manner in which oxygen is supplied, which is accomplished here by mechanical aerators at the surface.
(c) Percolating filters (also called bacterial beds).
They consist of a bed of stones 10 cm. in diameter with a depth of from 1 to 3 meters, occupying a large area over which, with the aid of a rotary arm, sewage is deposited on the upper part and is purified by the action of the microorganisms present as it flows down.
(d) Sludge activators (also called slush activators)
This consists of aeration in a rectangular tank (5 to 10 hours), followed by a circular sedimentation and contact tank (5 to 15 days). In the first tank there is rapid biological growth due to the oxygen in the air and agitation of the water. In the second tank, mineralization of the organic substance is complete as is the sedimentation of the biological substance (biomass) which is then recirculated to the first tank to make processing more efficient.
4. Tertiary treatment
After the sewage has been subjected to prior treatments, primary and secondary, a high level of products which have not been eliminated by the previous treatments usually remain in solution. These products are usually ionic or molecular. The first category includes soluble mineral salts (heavy and toxic metals) and the second, organic products of high molecular weight (non-biodegradable compounds).
The goal of this tertiary treatment is the elimination of these products. Tertiary treatment can be accomplished by:
5. Disinfection
Water is said to have undergone disinfection treatment when all pathogenic germs have been eliminated.
There are various chemicals which can be used to this end depending on the characteristics of the water to be purified.
Examples: Potassium permanganate; Ozone; Chlorine and its derivatives, and others known to those skilled in the art.
Hydrogen peroxide, which is used in the method of the present invention, is currently used, for example, by the firm of Foret, S.A. to eliminate hydrosulfuric acid and has also been used experimentally by Bayer, S.A. to eliminate hydrazine. Foret, Inc. mixes hydrogen peroxide with sewage and waits until a reaction occurs between the peroxide and the hydrosulfuric acid. They do not use a catalyst. On the other hand, Bayer, AG does not indicate when or where the mixture is made, but they do use a catalyst so the hydrogen peroxide decomposes. In this case the catalyst is a palladium resin. Results obtained are not satisfactory.